A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for various applications. Fuel cells are electrochemical devices which combine a fuel such as hydrogen and an oxidant such as oxygen from air to produce electricity and a water byproduct. In particular, individual fuel cells can be stacked together in series to form a fuel cell stack capable of supplying a quantity of electricity sufficient to power an electric vehicle. The fuel cell stack has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell stack. The PEM fuel cell typically includes three basic components: a cathode electrode, an anode electrode, and an electrolyte membrane. The electrolyte membrane is sandwiched between the cathode and the anode, which in turn is sandwiched between conductive, gas permeable diffusion media or diffusion layers. The diffusion media serve as current collectors for the anode and the cathode as well as provide mechanical support for the fuel cell. The diffusion media, the electrodes, and the electrolyte membrane are generally pressed between a pair of electronically conductive plates that distribute the fuel and the oxidant to the electrodes and complete the PEM fuel cell.
The fuel cell stack generally has a desired temperature range within which the fuel cell stack operation is optimized. To maintain the temperature of the fuel cell stack within the desired temperature range under normal operation, a coolant system that circulates a coolant and radiates excess heat from the fuel cell stack is typically employed. However, during a start-up operation of the fuel cell stack under cold conditions, the fuel cell stack temperature must be raised to the desired range. It is known to heat the coolant during the start-up operation with a coolant heater, for example. The coolant heater includes resistive electrical heating elements. The heated coolant is used to raise the temperature of the fuel cell stack. The resistive electrical heating elements are disengaged when the fuel cell stack generates a sufficient quantity of heat to maintain the desired temperature in conjunction with the coolant system. The temperature of the fuel cell stack is thereby regulated as desired.
Charge air coolers (CAC) for transferring heat from hot compressed air, such as the air supplied as the oxidant to the fuel cell stack, are also known. Typically, the coolant heaters and the charge air coolers operate independently of one another. Another known heat exchanger system for heating air is disclosed in EP1621378 to Brun et al., herein incorporated by reference in its entirety. EP1621378 describes a heating arrangement including self-regulating heating elements such as at least one positive temperature coefficient (PTC) element arranged between waved metal ribs for heating of air supplied to an automotive cabin.
The coolant heaters and the charge air coolers are expensive and can be volumetrically inefficient, often requiring considerable packaging space within an interior of the electric vehicle. The employment of such systems undesirably adds to complexity in designing and manufacturing the fuel cell powerplant. Conventional resistive electrical heating elements also undesirably require additional componentry in order to militate against an overheating of the coolant supplied to the fuel cell stack.
There is a continuing need for an integrated charge air heat exchanger that is volumetrically efficient, minimizes fuel cell system design and manufacturing complexity, and reduces fuel cell system cost. Desirably, the integrated charge air heat exchanger combines the coolant heater and the charge air cooler functions into a single unit, and employs self-regulating heating elements to heat a coolant in a first operating mode, and heat or cool a charge air stream in a second operating mode.